Beginner Question: How to close/split panels?
You can use the mouse pointer too, as you probably saw in those videos. Move the cursor around the corner of one of two equally sized adjacent panels, when it becomes a cross. 

Then click and keep the mouse button pressed, then drag up/down (to join panels vertically) or left/right (to join panels horizontally), see the animation below for some examples:



As you see, you can also split panels in this way: just start the dragging from a panel and move inward, that panel will split in two.